In the process of chlorophyll photosynthesis CO2 is an essential component. In nature air contains 300 or 400 parts CO2 per million parts and is constantly supplied (by the wind). The marijuana plants take the CO2 out of the air very quick. If the amount of CO2 is reduced by a third, there will not be any photosynthesis so also no growth.
The exhaust system makes sure CO2 comes in with fresh air and gets rid of the warm air with too little CO2 in it. Because CO2 is heavier then O2, most of it is close to the ground. make sure that the fans mix the air so the CO2 can reach the marijuana plants. If you do not add extra CO2 to a closed grow environment it is necessary to continuously let the exhaust and ventilation system work while the lights are on. This will provide good conditions for photosynthesis and thus for development.
CO2, temperature, water and fertilizers could all be limiting factors. Not enough of one of the elements could slow the whole development of the marijuana plants down. With a higher level of CO2 in the air, the metabolism of the plant goes faster. If it rises to 1500 ppm, it will have to go with a temperature of 25- 30 degrees Celsius. Also the minimum humidity goes up from 40 to 60% and even the amount of fertilizers changes. If this is all done correctly though, the harvest can be 15 to 25% bigger in the same time (or less!).
If you want a strong marijuana plant with flowers close together and a short growth it is wise to artificially add CO2. This technique is very difficult so only experienced growers should try it.
When to use artificial supplements of CO2:
When you have a heat problem and the exhaust system doesn't work properly you could try using artificial CO2. Everything accelerates when you do this, so if it's not strictly necessary. Don't do this the first few times you grow marijuana. When you are more experienced it can increase productivity and gain time.
CO2: not easy to measure.
The devices you need to measure the CO2- level are very expensive, although there is a chemical measure process but you can only do this once with the equipment: a glass tube divided into degrees and a syringe. The tube is filled with a reactive substance for CO2. The syringe is filled with the air that you want to test. Break the tube at the two ends and attach it to the syringe. The air should flow through the tube and the reactive substance changes color: this indicates the CO2 level.